1. Field of the Invention
The present invention relates to excitation circuits for operation of gas discharge tubes, particularly neon lamps.
2. Technological Background
A neon lamp is an illumination device formed from a sealed glass tube containing an ionizable gas, such as neon or a combination of argon and mercury. It has electron emitting cathodes and electrical terminals at each end of the tube. Upon application of the appropriate electrical signal to the terminals, the gas ionizes and a glow discharge is supported through the tube between cathode and terminal. Different colors of light are emitted from the lamp depending upon the composition of the gas in the tube.
Neon lamps are usually driven by alternating current ("A.C.") power sources. Commercial electrical supplies are commonly used as a local source of power. In North America, electricity is supplied at a frequency of 60Hz and at a voltage typically between 110 to 120 volts. Increased voltages are required for exciting neon lamps and have been provided by using step up transformers between the line outlet supplying the commercial power and the lamp. However, operation of neon lamps at the low 60Hz frequency of commercial power results in an annoying level of low pitched noise from the transformer and requires the use of relatively bulky transformer components.
Recent excitation circuits for neon lamps, and other discharge devices, have utilized relatively high speed switching circuits called inverters to produce A.C. at frequencies in the area of several tens of thousands of Hertz. Low frequency commercial A.C. is converted to direct current ("D.C.") by a rectifier. A primary winding of a step up transformer and a solid state transistor switch are connected in series across the output terminals of the rectifier. The transistor switch, in part, provides the inverter element. The switch is driven in and out of conduction at the desired operating frequency, producing a high frequency A.C. in the primary winding of the transformer.
Application of A.C. at a frequency in excess of 15,000 to 20,000Hz to the primary winding of the step up transformer pushes the frequency of consequential transformer noise to a level beyond the threshold of human hearing. High frequency operation also allows a decrease in the bulk of transformer components, reduces heat loss from the transformer during operation and reduces the need for bulky structural support for a transformer. Additionally, operation of neon lamps at high frequency is more efficient, with energy consumption per lumen of light generated being reduced.
Good output power control from the excitation circuit requires that the peak voltage level between the switch and the primary winding of the transformer have a constant maximum. Output power control is important to insure proper operation of the lamp and to permit use of minimum cost components. Commercial A.C. is subject to peak and root mean square ("r.m.s.") voltage level excursions. Consequentially, the peak voltage level between the switch and the transformer primary winding will also be subject to excursions unless the circuit is capable of adjusting for such excursions. Load changes across the secondary winding affect the peak voltage across the switch to an even greater extent. Upward excursions of the peak voltage level can result in destruction of a solid state transistor switch.